This invention relates to meter boxes for housing meters for measuring the components and characteristics of electric power supplied to a load, and more specifically to a meter box of this type having improved safety features for eliminating the possibility of electrical shock when a hook-on type ammeter is utilized to measure the multi-phase current to the load.
It is well known in the heavy electrical industry to utilize a hook-on ammeter of the type having a hook shaped magnetic core probe which is so constructed to permit the probe to be hooked around a current carrying wire or cable. The magnetic core probe of hook-on ammeters of this type reacts with the magnetic field generated by the current flowing through the wire, the strength of the magnetic field being proportional to the current through the wire. As a result, accurate current measurements may be made without the necessity of breaking the wire and inserting an ammeter therein.
In multi-phase applications, as with heavy industrial electric motors, the different currents in the different phases produce corresponding magnetic fields that always cancel each other, with the result that no magnetic field is generated around a multi-phase power cable. Therefore, prior to the instant invention, it has been impossible to measure multi-phase currents by using hook-on type ammeters.